A universal mobile telecommunication system (UMTS) is a European-type, third generation IMT-2000 mobile communication system that has evolved from a European standard known as Global System for Mobile communications (GSM). UMTS is intended to provide an improved mobile communication service based upon a GSM core network and wideband code division multiple access (W-CDMA) wireless connection technology. In December 1998, a Third Generation Partnership Project (3GPP) was formed by the ETSI of Europe, the ARIB/TTC of Japan, the T1 of the United States, and the TTA of Korea. The 3GPP creates detailed specifications of UMTS technology.
In order to achieve rapid and efficient technical development of the UMTS, five technical specification groups (TSG) have been created within the 3GPP for standardizing the UMTS by considering the independent nature of the network elements and their operations. Each TSG develops, approves, and manages the standard specification within a related region. The radio access network (RAN) group (TSG-RAN) develops the standards for the functions, requirements, and interface of the UMTS terrestrial radio access network (UTRAN), which is a new radio access network for supporting W-CDMA access technology in the UMTS.
FIG. 1 provides an overview of a UMTS network. The UMTS network includes a mobile terminal or user equipment (UE) 1, a UTRAN 2 and a core network (CN) 3.
The UTRAN 2 includes several radio network controllers (RNCs) 4 and NodeBs 5 that are connected via the Iub interface. Each RNC 4 controls several NodeBs 5. Each NodeB 5 controls one or several cells, where a cell covers a given geographical area on a given frequency.
Each RNC 4 is connected via the Iu interface to the CN 3 or towards the mobile switching center (MSC) 6 entity of the CN and the general packet radio service (GPRS) support Node (SGSN) 7 entity. RNCs 4 can be connected to other RNCs via the Iur interface. The RNC 4 handles the assignment and management of radio resources and operates as an access point with respect to the CN 3.
The NodeBs 5 receive information sent by the physical layer of the UE 1 via an uplink and transmit data to the UE 1 via a downlink. The Node-Bs 5 operate as access points of the UTRAN 2 for the UE 1.
The SGSN 7 is connected to the equipment identity register (EIR) 8 via the Gf interface, to the MSC 6 via the GS interface, to the gateway GPRS support node (GGSN) 9 via the GN interface, and to the home subscriber server (HSS) via the GR interface.
The EIR 8 hosts lists of UEs 1 that are available for use on the network.
The MSC 6, which controls the connection for circuit switched (CS) services, is connected towards the media gateway (MGW) 11 via the NB interface, towards the EIR 8 via the F interface, and towards the HSS 10 via the D interface.
The MGW 11 is connected towards the HSS 10 via the C interface and also to the public switched telephone network (PSTN). The MGW 11 also allows the codecs to adapt between the PSTN and the connected RAN.
The GGSN 9 is connected to the HSS 10 via the GC interface and to the Internet via the GI interface. The GGSN 9 is responsible for routing, charging and separation of data flows into different radio access bearers (RABs). The HSS 10 handles the subscription data of users.
The UTRAN 2 constructs and maintains an RAB for communication between a UE 1 and the CN 3. The CN 3 requests end-to-end quality of service (QoS) requirements from the RAB and the RAB supports the QoS requirements set by the CN 3. Accordingly, the UTRAN 2 can satisfy the end-to-end QoS requirements by constructing and maintaining the RAB.
The services provided to a specific UE 1 are roughly divided into CS services and packet switched (PS) services. For example, a general voice conversation service is a CS service and a Web browsing service via an Internet connection is classified as a PS service.
The RNCs 4 are connected to the MSC 6 of the CN 3 and the MSC is connected to the gateway MSC (GMSC) that manages the connection with other networks in order to support CS services. The RNCs 4 are connected to the SGSN 7 and the gateway GGSN 9 of the CN 3 to support PS services.
The SGSN 7 supports packet communications with the RNCs. The GGSN 9 manages the connection with other packet switched networks, such as the Internet.
FIG. 2 illustrates a structure of a radio interface protocol between a UE 1 and the UTRAN 2 according to the 3GPP radio access network standards. As illustrated In FIG. 2, the radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data and a control plane (C-plane) for transmitting control information. The U-plane is a region that handles traffic information with the user, such as voice or Internet protocol (IP) packets. The C-plane is a region that handles control information for an interface with a network as well as maintenance and management of a call. The protocol layers can be divided into a first layer (L1), a second layer (L2), and a third layer (L3) based on the three lower layers of an open system interconnection (OSI) standard model.
The first layer (L1), or physical layer, provides an information transfer service to an upper layer by using various radio transmission techniques. The physical layer is connected to an upper layer, or medium access control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel.
The second layer (L2) includes a MAC layer, a radio link control (RLC) layer, a broadcast/multicast control (BMC) layer, and a packet data convergence protocol (PDCP) layer. The MAC layer handles mapping between logical channels and transport channels and provides allocation of the MAC parameters for allocation and re-allocation of radio resources. The MAC layer is connected to an upper layer, or the radio link control (RLC) layer, via a logical channel.
Various logical channels are provided according to the type of information transmitted. A control channel is generally used to transmit information of the C-plane and a traffic channel is used to transmit information of the U-plane. A logical channel may be a common channel or a dedicated channel depending on whether the logical channel is shared.
FIG. 3 illustrates the different logical channels that exist. Logical channels include a dedicated traffic channel (DTCH), a dedicated control channel (DCCH), a common traffic channel (CTCH), a common control channel (CCCH), a broadcast control channel (BCCH), and a paging control channel (PCCH), or a Shared Control Channel (SCCH), as well as other channels. The BCCH provides information including information utilized by a UE 1 to access a system. The PCCH is used by the UTRAN 2 to access a UE 1.
Additional traffic and control channels are introduced in the Multimedia Broadcast Multicast Service (MBMS) standard for the purposes of MBMS. The MBMS point-to-multipoint control channel (MCCH) is used for transmission of MBMS control information. The MBMS point-to-multipoint traffic channel (MTCH) is used for transmitting MBMS service data. The MBMS scheduling channel (MSCH) is used to transmit scheduling information.
The MAC layer is connected to the physical layer by transport channels. The MAC layer can be divided into a MAC-b sub-layer, a MAC-d sub-layer, a MAC-c/sh sub-layer, a MAC-hs sub-layer and a MAC-m sublayer according to the type of transport channel being managed.
The MAC-b sub-layer manages a broadcast channel (BCH), which is a transport channel handling the broadcasting of system information. The MAC-c/sh sub-layer manages a common transport channel, such as a forward access channel (FACH) or a downlink shared channel (DSCH), which is shared by a plurality of UEs 1, or in the uplink the radio access channel (RACH). The MAC-m sublayer may handle MBMS data.
FIG. 4 illustrates the possible mapping between the logical channels and the transport channels from a UE 1 perspective. FIG. 5 illustrates the possible mapping between the logical channels and the transport channels from a UTRAN 2 perspective.
The MAC-d sub-layer manages a dedicated channel (DCH), which is a dedicated transport channel for a specific UE 1. The MAC-d sublayer is located in a serving RNC 4 (SRNC) that manages a corresponding UE 1. One MAC-d sublayer also exists in each UE 1.
The RLC layer supports reliable data transmissions and performs segmentation and concatenation on a plurality of RLC service data units (SDUs) delivered from an upper layer depending of the RLC mode of operation. The RLC layer adjusts the size of each RLC SDU received from the upper layer in an appropriate manner based upon processing capacity and then creates data units by adding header information. The data units, or protocol data units (PDUs), are transferred to the MAC layer via a logical channel. The RLC layer includes a RLC buffer for storing the RLC SDUs and/or the RLC PDUs.
The BMC layer schedules a cell broadcast (CB) message transferred from the CN 3. The BMC layer broadcasts the CB message to UEs 1 positioned in a specific cell or cells.
The PDCP layer is located above the RLC layer. The PDCP layer is used to transmit network protocol data, such as the IPv4 or IPv6, efficiently on a radio interface with a relatively small bandwidth. The PDCP layer reduces unnecessary control information used in a wired network, a function called header compression, for this purpose.
The radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the C-plane. The RRC layer controls the transport channels and the physical channels in relation to setup, reconfiguration, and the release or cancellation of the radio bearers (RBs).
A RB signifies a service provided by the second layer (L2) for data transmission between a UE 1 and the UTRAN 2. The set up of the RB generally refers to the process of stipulating the characteristics of a protocol layer and a channel required for providing a specific data service and setting the respective detailed parameters and operation methods. The RRC also handles user mobility within the RAN and additional services, such as location services.
Not all different possibilities for the mapping between the RBs and the transport channels for a given UE 1 are available all the time. The UE 1/UTRAN 2 deduce the possible mapping depending on the UE state and the procedure presently executed by the UE/UTRAN.
The different transport channels are mapped onto different physical channels. The configuration of the physical channels is given by RRC signaling exchanged between the RNC 4 and the UE 1.
Initial access is a procedure whereby a UE 1 sends a first message to the UTRAN 2 using a common uplink channel, specifically the Random Access Channel (RACH). For both GSM and UMTS systems, the initial access procedure involves the UE 1 transmitting a connection request message that includes a reason for the request and receiving a response from the UTRAN 2 indicating the allocation of radio resources for the requested reason.
There are several reasons, or establishment causes, for sending a connection request message. Table I indicates the establishment causes specified in UMTS, specifically in 3GPP TS 25.331.
TABLE 1Establishment CausesOriginating Conversational CallOriginating Streaming CallOriginating Interactive CallOriginating Background CallOriginating Subscribed traffic CallTerminating Conversational CallTerminating Streaming CallTerminating Interactive CallTerminating Background CallEmergency CallInter-RAT cell re-selectionInter-RAT cell change orderRegistrationDetachOriginating High Priority SignalingOriginating Low Priority SignalingCall re-establishmentTerminating High Priority SignalingTerminating Low Priority Signaling
The “originating call” establishment cause indicates that the UE 1 wants to setup a connection, for example, a speech connection. The “terminating call” establishment cause indicates that that UE 1 answers to paging. The “registration” establishment cause indicates that that the user wants to register only to the network.
A physical random access procedure is used to send information over the air. The physical random access transmission is under control of a higher layer protocol, which performs important functions related to priority and load control. This procedure differs between GSM and UMTS radio systems.
The description of GSM random access procedure can be found in “The GSM System for Mobile Communications” published by M. Mouly and M. B. Pautet, 1992. As the present invention is related to UMTS enhancement and evolution, the W-CDMA random access procedure is detailed herein. Although the present invention is explained in the context of UMTS evolution, the present invention is not so limited.
The transport channel RACH and two physical channels, Physical Random Access Channel (PRACH) and Acquisition Indication Channel (AICH), are utilized in this procedure. The transport channels are channels supplied by the physical layer to the protocol layer of the MAC layer. There are several types of transport channels to transmit data with different properties and transmission formats over the physical layer.
Physical channels are identified by code and frequency in Frequency Division Duplex (FDD) mode and are generally based on a layer configuration of radio frames and timeslots. The form of radio frames and timeslots depends on the symbol rate of the physical channel.
A radio frame is the minimum unit in the decoding process, consisting of 15 time slots. A time slot is the minimum unit in the Layer 1 bit sequence. Therefore, the number of bits that can be accommodated in one time slot depends on the physical channel.
The transport channel RACH is an uplink common channel used for transmitting control information and user data. The transport channel RACH is utilized in random access and used for low-rate data transmissions from a higher layer. The RACH is mapped to an uplink physical channel, specifically the PRACH. The AICH is a downlink common channel, which exists as a pair with PRACH used for random access control.
The transmission of PRACH is based on a slotted ALOHA approach with fast acquisition indication. The UE randomly selects an access resource and transmits a RACH preamble part of a random access procedure to the network.
A preamble is a short signal that is sent before the transmission of the RACH connection request message. The UE 1 repeatedly transmits the preamble by increasing the transmission power each time the preamble is sent until it receives the Acquisition Indicator (AI) on AICH, which indicates the detection of the preamble by the UTRAN 2. The UE 1 stops the transmission of the preamble once it receives the AI and sends the message part at the power level equal to the preamble transmission power at that point, adding an offset signaled by the UTRAN 2. FIG. 6 illustrates a power ramping procedure.
This random access procedure avoids a power ramping procedure for the entire message. A power ramping procedure would create more interference due to unsuccessfully sent messages and would be less efficient due to a larger delay since it would take much more time to decode the message before an acknowledgement could be transmitted to indicate successful receipt of the message.
The main characteristics of the RACH is that it is a contention based channel subject to collisions due to simultaneous access of several users, which may preclude decoding of the initial access message by the network. The UE 1 can start the random access transmission of both preambles and message only at the beginning of an access slot. This access method is, therefore, a type of slotted ALOHA approach with fast acquisition indication
The time axis of both the RACH and the AICH is divided into time intervals or access slots. There are 15 access slots per two frames, with each frame having a length of 10 ms or 38400 chips, and the access slots are spaced 1.33 ms or 5120 chips apart. FIG. 7 illustrates the number and spacing of access slots.
The UTRAN 2 signals information regarding which access slots are available for random access transmission and the timing offsets to use between RACH and AICH, between two successive preambles and between the last preamble and the message. For example, if the AICH transmission timing is 0 and 1, it is sent three and four access slots after the last preamble access slot transmitted, respectively. FIG. 8 illustrates the timing of the preamble, AI and message part
The timing at which the UE 1 can send the preamble is divided according to random access sub channels. A random access sub channel is a subset including the combination of all uplink access slots. There are 12 random access sub channels. A random access sub channel consists of the access slots indicated in Table II.
TABLE 2SFN modulo 8 ofcorrespondingSub-channel numberP-CCPCH frame012345678910110012345671121314891011201234567391011121314846701234558910111213146345670127891011121314
The preamble is a short signal that is sent before the transmission of the RACH message. A preamble consists of 4096 chips, which is a sequence of 256 repetitions of Hadamard codes of length 16 and scrambling codes assigned from the upper layer.
The Hadamard codes are referred to as the signature of the preamble. There are 16 different signatures and a signature is randomly selected from available signature sets on the basis of Access Service Classes (ASC) and repeated 256 times for each transmission of the preamble part. Table III lists the preamble signatures.
The message part is spread by Orthogonal Variable Spreading Factor (OVSF) codes that are uniquely defined by the preamble signature and the spreading codes for use as the preamble signature. The 10 ms long message part radio frame is divided into 15 slots, each slot consisting of 2560 chips.
TABLE 3PreambleValue of nsignature0123456789101112131415P0(n)1111111111111111P1(n)1−11−11−11−11−11−11−11−1P2(n)11−1−111−1−111−1−111−1−1P3(n)1−1−111−1−111−1−111−1−11P4(n)1111−1−1−1−11111−1−1−1−1P5(n)1−11−1−11−111−11−1−11−11P6(n)11−1−1−1−11111−1−1−1−111P7(n)1−1−11−111−11−1−11−111−1P8(n)11111111−1−1−1−1−1−1−1−1P9(n)1−11−11−11−1−11−11−11−11P10(n)11−1−111−1−1−1−111−1−111P11(n)1−1−111−1−11−111−1−111−1P12(n)1111−1−1−1−1−1−1−1−11111P13(n)1−11−1−11−11−11−111−11−1P14(n)11−1−1−1−111−1−11111−1−1P15(n)1−1−11−111−1−111−11−1−11
Each slot includes a data part and a control part that transmits control information, such as pilot bits and TFCI. The data part and the control part are transmitted in parallel. The 20 ms long message part consists of two consecutive message part radio frames. The data part consists of 10*2 k bits, where k=0, 1, 2, 3, which corresponds to a Spreading Factor (SF) of 256, 128, 64, 32. FIG. 9 illustrates the structure of the random access message part.
The AICH consists of a repeated sequence of 15 consecutive access slots, each slot having a length of 40 bit intervals or 5120 chips. Each access slot includes two parts, an Acquisition Indicator (AI) part consisting of 32 real-valued signals, such as a0 . . . a31, and a part having a length of 1024 chips during which transmission is switched off. FIG. 10 illustrates the structure of the AICH.
When the UTRAN 2 detects transmission of a RACH preamble having a certain signature in an RACH access slot, the UTRAN repeats this signature in the associated AICH access slot. Therefore, the Hadamard code used as the signature for the RACH preamble is modulated onto the AI part of the AICH.
The acquisition indicator corresponding to a signature can have a value of +1, −1 or 0 depending on whether a positive acknowledgement (ACK), a negative acknowledgement (NACK) or no acknowledgement is received in response to a specific signature. The positive polarity of the signature indicates that the preamble has been acquired and the message can be sent.
The negative polarity indicates that the preamble has been acquired and the power ramping procedure shall be stopped, but the message shall not be sent. This negative acknowledgement is used when a received preamble cannot be processed at the present time due to congestion in the UTRAN 2 and the UE 1 must repeat the access attempt some time later.
All UEs 1 are members of one of ten randomly allocated mobile populations, defined as Access Classes (AC) 0 to 9. The population number is stored in the Subscriber Identity Module (SIM)/Universal Subscriber Identity Module (USIM). UEs 1 may also be members of one or more out of 5 special categories of Access Classes 11 to 15, which are allocated to specific high priority users and the information also stored in the SIM/USIM. Table IV lists the special AC and their allocation.
TABLE 4ACAllocation15PLMN Staff14Emergency Services13Public Utilities (e.g. water/gas suppliers)12Security Services11For PLMN Use
The UTRAN 2 performs the random access procedure at protocol layer L2 by determining whether to permit the UE 1 to use a radio access resource based primarily upon the AC to which the UE belongs.
It will be desirable to prevent UE 1 users from making access attempts, including emergency call attempts, or responding to pages in specified areas of a Public Land Mobile Network (PLMN) under certain circumstances. Such situations may arise during states of emergency or where 1 or more co-located PLMNs has failed. Broadcast messages should be available on a cell-by-cell basis to indicate the class(es) of subscribers barred from network access. The use of this facility allows the network operator to prevent overload of the access channel under critical conditions
Access attempts are allowed if the UE 1 is a member of at least one AC that corresponds to the permitted classes as signaled over the air interface and the AC is applicable in the serving UTRAN 2. Access attempts are otherwise not allowed. Any number of these AC may be barred at any one time. Access Classes are applicable as indicated in Table V.
TABLE 5ACApplicability0-9Home and Visited PLMNs11 and 15Home PLMN only12, 13, 14Home PLMN and visited PLMNs of home country only
An additional control bit for AC 10 is also signaled over the air interface to the UE 1. This control bit indicates whether access to the UTRAN 2 is allowed for Emergency Calls for UEs 1 with access classes 0 to 9 or without an International Mobile Subscriber Identity (IMSI). Emergency calls are not allowed if both AC 10 and the relevant AC, 11 to 15 are barred for UEs 1 with access classes 11 to 15. Emergency calls are otherwise allowed.
The AC are mapped to ASC In the UMTS. There are eight different priority levels defined, specifically ASC 0 to ASC 7, with level 0 representing the highest priority.
Access Classes shall only be applied at initial access, such as when sending an RRC Connection Request message. A mapping between AC and ASC shall be indicated by the information element “AC-to-ASC mapping” in System Information Block type 5. The correspondence between AC and ASC is indicated in Table VI.
TABLE 6AC0-9101112131415ASC1st IE2nd IE3rd IE4th IE5th IE6th IE7th IE
In Table VI, “nth IE” designates an ASC number i in the range 0-7 to AC. The UE 1 behavior is unspecified if the ASC indicated by the “nth IE” is undefined.
The parameters implied by the respective ASC are utilized for random access. A UE 1 that is a member of several ACs selects the ASC for the highest AC number. The AC is not applied in connected mode.
An ASC consists of a subset of RACH preamble signatures and access slots that are allowed for the present access attempt and a persistence value corresponding to a probability, Pv≦1, to attempt a transmission. Another important mechanism to control random access transmission is a load control mechanism that reduces the load of incoming traffic when the collision probability is high or when the radio resources are low.
The physical layer (L1) random access procedure is initiated upon request from the MAC sub layer (L2). The physical layer receives information from a higher layer, specifically the RRC, before the physical random-access procedure is initiated and receives information from a higher layer, specifically the MAC, at each initiation of the physical random access procedure. The information is indicated in Table VII. The physical layer random-access procedure is illustrated in FIG. 11.
As illustrated in FIG. 11, one access slot in the random access subchannel that can be used for the given ASC is randomly selected from access slots that can be used in the next full access slot sets (S200). One access slot is randomly chosen from access slots that can be used in the next full access slot sets if there are no access slots available. One signature is then randomly selected from the set of available signatures within the given ASC (S210).
TABLE 7Information Related to Physical Random-Access ProcedureBefore Initiation of ProcedureUpon Initiating ProcedurePreamble scrambling code.Transport Formatfor PRACHmessage part.Message length in time (10 or 20 ms)ASC of the PRACHtransmissionAICH_Transmission_Timing parameter (0Data to be transmittedor 1)(Transport Block Set)Set of available signatures and set ofavailable RACH sub-channels for eachAccess Service Class (ASC).Power-ramping factor Power Ramp Step(integer > 0)Preamble Retrans Max parameter (integer >0)Initial preamble power(Preamble_Initial_Power)Power offset in dB between power of thelast transmitted preamble and power of thecontrol part of the random-access message(Pp-m = Pmessage-control − Ppreamble measured)Set of Transport Format parameters(including power offset between the datapart and the control part of the random-access message for each TransportFormat)
The preamble retransmission counter is set at Preamble Retrans Max (S220), which is the maximum number of preamble retransmission attempts. The preamble transmission power is set at Preamble Initial Power (S230), which is the initial transmission power of the preamble. The preamble is then transmitted according to the chosen uplink access slot, signature and set transmission power (S240).
The UE 1 then determines whether the UTRAN 2 detected the preamble (S250). No random access message is transmitted if a NACK is detected in the downlink access slot corresponding to the selected uplink access slot. A random access message is transmitted if an ACK is detected in the downlink access slot corresponding to the selected uplink access slot. The preamble is retransmitted if no response, specifically neither an ACK nor a NACK for the selected signature, is detected in the downlink access slot corresponding to the selected uplink access slot.
When no response is received, the next available access slot is selected from the random access subchannel within the given ASC (S260), a new signature is randomly selected from the available signatures within the given ASC (S270), the preamble transmission power is increased by the step width of the power ramping (Power Ramp Step) (S280) and the preamble retransmission counter is reduced by 1 (S290). The UE 1 then determines if the maximum number of retransmissions have been attempted (S300). This preamble re-transmission procedure is repeated for as long as the preamble retransmission counter exceeds 0 and no response is received. The MAC is informed that no ACK was received on AICH (S310) and the physical layer random access procedure is terminated once the retransmission counter reaches 0.
If an ACK is received, the transmission power of the control channel of the random access message is set at a level higher than the transmission power of the last preamble transmitted according to a power offset (S320) and the random access message is transmitted 3 or 4 uplink access slots after the uplink access slot of the last transmitted preamble depending on the AICH transmission timing parameter (S330). The higher layer is then informed of the receipt of the ACK and transmission of the random access message (S340) and the physical layer random access procedure is terminated.
If a NACK is received, no random access message is transmitted and no re-transmission of the preamble is performed. The MAC is informed that a NACK was received (S350) and the physical layer random access procedure is terminated.
FIG. 12 illustrates a signaling establishment procedure between a UE 1 and UTRAN 2. As illustrated in FIG. 12, the RRC Connection Request message is transmitted once the PRACH power control preambles have been acknowledged (S400). The RRC Connection Request message includes a reason for requesting the connection.
The UTRAN 2 determines which resources to reserve and performs synchronization and signaling establishment among radio network nodes, such as a NodeB 5 and serving RNC 4, depending on the request reason (S410). The UTRAN 2 then transmits the Connection Setup message to the UE 1, thereby conveying information about radio resource to use (S420).
The UE 1 confirms connection establishment by sending the Connection Setup Complete message to the UTRAN 2 (S430). The UE 1 transmits the Initial Direct Transfer message to the UTRAN 2 once the connection has been established (S440). The Initial Direct Transfer message includes information such as the UE identity, UE current location and the kind of transaction requested.
Authentication is then performed between the UE 1 and UTRAN 2 and security mode communication is established (S450). The actual set up information is delivered to the UTRAN 2 from the UE 1 via the Call Control Setup message (S460). The Call Control Setup message identifies the transaction and indicates the QoS requirements.
The UTRAN 2 initiates activities for radio bearer allocation by determining if there are sufficient resources available to satisfy the requested QoS and transmits the Call Control Complete message to the UE 1 (S470). The radio bearer is allocated according to the request if there are sufficient resources available. The UTRAN 2 may select either to continue allocation with a lowered QoS value, queue the request until sufficient radio resources become available or reject the call request if sufficient resources are not presently available.
FIG. 13 illustrates the architecture of an LTE system. Each aGW 115 is connected to one or several access Gateways (aGW) 115. An aGW 115 is connected to another Node (not shown) that allows access to the Internet and/or other networks, such as GSM, UMTS, and WLAN.
The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement. Generally, The UTRAN 2 corresponds to E-UTRAN (Evolved-UTRAN). The NodeB 5 and/or RNC 4 correspond to e-NodeB (eNB) in the LTE system.
Hybrid ARQ (HARQ) allows optimizing the use of re-transmitted packets. The principle of HARQ is that the receiver indicates to the transmitter whether or not the decoding of a packet was successful. The transmitter re-transmits the same data when the decoding was not successful such that the receiver can combine the earlier received packet with the newly received packet. The re-transmitted data may be coded in a different way. Therefore, HARQ re-uses the information of the earlier received packet in order to decode the transmitted data
HARQ implies that the transmitter indicates on a first shared control channel to which receiver a transmission is intended if the channel is shared, as is proposed for LTE or as is done in UMTS with the High Speed Downlink Packet Access (HSDPA) channel. Each UE 1 is assigned a unique Cell Radio Network Temporary Identifier (C-RNTI) for normal operation in order to identify different users on the shared control channel (SCCH).
Information received on the SCCH indicates whether the transmission is a first transmission or whether the transmission is related to an earlier unsuccessful transmission, and possibly to which earlier transmission a re-transmission is related. The control information further indicates other information, such as coding schemes, redundancy versions and resources used for the transmission.
The receiver detects that it is the intended target for a transmission it has received based on the control information and receives the packet. If applicable, the receiver combines the received packets and decodes the message.
The random access procedure is outlined in FIG. 14 and includes four steps. The procedure is initiated for different reasons, such as to contact the eNB to trigger a first transmission, to receive timing alignment of the UE 1 uplink transmission with the time reference in the eNB, and to request uplink transmission.
In the first step illustrated in FIG. 14, the UE 1 selects a Random Access Preamble on RACH and transmits the selected preamble in uplink message 1. The preamble is selected from of one or several sets of possible preambles based on two factors. The first factor is the cause of the access or size of information to transmit, potentially with priority. The second factor is pathloss or channel quality indication (CQI), which indicates the uplink and downlink channel quality measurements, in order to allocate uplink resources appropriately.
In the second step illustrated in FIG. 14, the network transmits a Random Access Response (message 2) to the UE 1 when the network has detected the preamble. The UE 1 will restart the random access procedure at step 1 if message 2 is not received within a certain time.
The transmission of message 2 in the downlink is semi-synchronous with the transmission of message 1 in step 1 in that it occurs within a flexible window of which the size is one or more transmission time intervals (TTI). No HARQ is used for the transmission of message 2, which is transmitted on a layer L1/L2 DL-SCH and addressed with the Random Access Radio Network Temporary Identifier (RA-RNTI) on the L1/L2 control channel.
Message 2 is intended for one or more UEs 1 in one DL-SCH message and conveys at least the Random Access (RA) preamble identifier, timing alignment information, initial uplink grant and assignment of a temporary C-RNTI. The temporary C-RNTI may be made permanent upon the Contention Resolution of message 4. The temporary C-RNTI is not unique and may be used by several UEs 1 if more than one UE simultaneously transmit the preamble message 1 in step 1.
A C-RNTI is an identifier that provides a unique UE 1 identification at the cell level, which is normally 16 bits, in order to identify an RRC connection between the UE and the network. The UE 1 will have to change its C-RNTI when changing a cell.
In the third step illustrated in FIG. 14, the transmission of message 3 on UL-SCH is the first scheduled uplink transmission, such as an RRC Connection Request for initial access. HARQ is used for the transmission of message 3, which is generated at the RLC layer with no segmentation.
Message 3 has a dynamic size and conveys at least a UE identifier, such as a private C-RNTI if available, International Mobile Equipment Identifier (IMEI), Temporary Mobile Subscriber Identifier (TMSI), or International Mobile Subscriber Identifier (IMSI) message or information facilitating formation of the initial Non-Access Stratum (NAS) message in the eNB 105 may be included in a message 3 transmitted for initial access if the message 3 size allows it.
In the fourth step illustrated in FIG. 14, message 4 is transmitted on DL-SCH for Contention Resolution, or RRC Contention Resolution for initial access. Message 4 is addressed to the temporary C-RNTI on the L1/L2 control channel at least for initial access and the transmission of message 4 is not synchronized with message 3.
The transmission of message 4 supports HARQ. HARQ feedback is transmitted only by the UE 1, which detects its own UE identity provided in message 3 and which is echoed in the RRC Contention Resolution message 4. The use of C-RNTI, HARQ and the related consequences, such as a delay impact on other UEs 1 in conjunction with HARQ, are possible for a UE in the RRC_CONNECTED state when message 4 is transmitted.
The UE 1 may consider that the access procedure is successful and determine that the timing advance, the temporary C-RNTI and the contents of message 4 are intended for it if the UE detects its UE identity as provided in message 3. The UE 1 discards information received in message 4 and restarts the initial access procedure if the UE does not correctly detect its UE identity in message 4.
FIG. 15 illustrates the assigning of the temporary C-RNTI using the RA-RNTI, which is a special C-RNTI reserved for the transmission of message 2. As illustrated in FIG. 15, the RA ?RNTI as well as other information is transmitted via the SCCH channel. The UE 1 then decides to receive the DL-SCH and will receive the temporary C-RNTI for later use if the reference to the signature corresponds to the signature used by the UE.
The temporary identifier included in message 2 normally represents 16 bits of information. However it is very costly to transmit this information because it has to be sent over the complete coverage area.
Contention in message 4 is detected by comparing the initial UE identifier in message 2 with the initial UE identity used in message 4. If two UEs 1 use two different identifiers, such that one UE uses the C-RNTI and another UE uses the TIMSI, it is possible that the C-RNTI used by the first UE and the TIMSI used by the other UE have the same value.